The study is designed to biochemically characterize the dense collagenous response to tumor invasion and by doing so, gain insight into the nature and purpose of this host response. Human breast cancer, because of its accessibility and because of its characteristic scirrhous or desmoplastic qualities will be the main tissue of investigation, but the study will be extended to other invasive tumors, which are, and are not associated with a desmoplastic response. Desmoplastic tissue is found to have a markedly increased content of type V collagen. It is proposed that myofibroblasts are recruited by mitogenic and chemotactic factors produced by the tumor cells. The myofibroblasts then contribute to the deposition of elastin and type V collagen. This hypothesis may extend to other situations in which unrestricted fibrosis may compromise host function.